Coral reefs face a multitude of threats, including overfishing, coastal development, pollution, and changes to ocean chemistry caused by rising levels of carbon dioxide in the atmosphere. But global warming presents a particularly grave threat because unusually warm ocean temperatures can lead to episodes of coral bleaching, in which corals turn white after expelling the colorful microscopic algae that provide them with nutrition. Unless cooler temperatures return in a few days or weeks, allowing algae to grow again, bleached corals often collapse and die.
Bleaching can occur naturally, but it has become increasingly widespread in recent decades. This is largely because sea-surface temperatures in tropical waters where corals live have increased about 0.5-0.7 degrees Fahrenheit (0.3-0.4 degrees Celsius) over the last two to three decades, with temperatures occasionally spiking higher.
However, between 1980 and 2005, only four episodes of bleaching have been reported for reefs in the Western Pacific Warm Pool. This is a lower rate than any other reef region, even though the western Pacific reefs appear to be especially sensitive to temperature changes. Sea-surface temperatures in the warm pool naturally average about 84 degrees Fahrenheit (29 degrees Celsius), which is close to the proposed thermostat limit. They have warmed up about half as much as in cooler areas of the oceans.
To study the correlation between temperatures and bleaching, the authors analyzed sea-surface temperatures from the period 1950-2006 in tropical waters that are home to corals, relying on measurements taken by ships, buoys, and satellites. They also used the NCAR-based Community Climate System Model to study computer simulations of past and future sea-surface temperatures. The team compared the actual and simulated temperatures to a database of coral bleaching reports, mostly taken from 1980 to 2005.
|Contact: David Hosansky|
National Center for Atmospheric Research/University Corporation for Atmospheric Research